1. Technical Field
This invention relates generally to a method and apparatus for actuating light sources, for example light emitting diodes, and more specifically to a method and apparatus for actuating a light source for illuminating a display or annunciator on an electronic device by staggering a plurality of pulse width modulated signals.
2. Background Art
Many electronic devices, including mobile telephones, personal digital assistants, and portable computers, include displays by which information is presented to a user. Many of these displays include lighting so that the display may be easily viewed in a dark environment. Some displays, like liquid crystal displays for instance, require the use of lighting for their operation regardless of the environment. Transmissive type liquid crystal displays include a variable translucent pixilated display and a backlight, such as a fluorescent lamp, light emitting diode, or other similar device, that projects light from behind the display. By selecting which pixels pass light and which do not, images are created on the display.
In many devices, multiple light sources may be used for backlighting. While some liquid crystal display televisions may employ a single bulb, smaller portable devices often use several light emitting diodes to illuminate their displays. One prior art method of illuminating the display is to turn on all of the light sources when the display is active, allowing them to remain on so long as information is active on the display. For example, where a person opens a flip-style telephone, the light sources may all come on and remain on until the telephone is closed.
The problem with this prior art solution is due to the fact that light sources consume power. Where the device is a battery-powered device, like a mobile telephone for example, energy consumed by light sources cannot be used in making telephone calls. The result is a shorter run time between battery recharges.
One prior art solution to this reduced run time problem is to pulse the light sources on and off while the display is active. As the human eye integrates rapidly passing images, rather than turning all the light sources on and leaving them on, the device may rapidly pulse the light sources on and off, on and off, and so forth. The net result is a display that looks illuminated to the human eye, but consumes less power than a continuously illuminated one.
The problem with this prior art solution is that turning multiple light sources on and off rapidly causes large current pulses to be drawn from the power supply. Where the power supply has an inherent, internal impedance, as is the case with a rechargeable battery, large instantaneous currents may cause the output voltage of the power source to fall. Thus, by actuating several light sources simultaneously, the supply voltage may dip or become erratic. Where the dips become significant, other operations within the device may be compromised. For example, dips in the supply voltage may cause undesirable flickering in the light sources themselves. Additionally, audio buzz, digital camera noise, communication problems, and other problems may be caused.
There is thus a need for an improved method and apparatus for illuminating displays and other devices within portable electronics.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.